Conventionally, a fan which includes an impeller having a plurality of blades powered by a motor is arranged inside an electronic device so as to cool heated components therein and to let out the heated air of the electronic device. In recent years, due to an increase in the amount of heat generated by the electronic device, the rotation speed of the impeller is increased.
When the rotation of the impeller is increased, a significant amount noise may be generated if a weight balance of a rotor assembly including a rotor portion (including the impeller) of the motor with respect to a rotational axis thereof is compromised. Also, when the rotation of the impeller is increased, the motor may be vibrated generating noise. In particular, the rotor holder of the rotor portion retaining thereat a field magnet is often made of a metal material, which is usually heavier than the material used to manufacture other elements of the motor, and therefore, the rotational balance of the rotor holder is a critical element of the operation of the motor.